"Saturday in the Park" is a song written by Robert Lamm and recorded by the group Chicago for their 1972 album Chicago V.

Based on this passage, who sings the song saturday in the park? Let's give stream of consciousness first:
The relevant information to answer the above question is: "Saturday in the Park" is a song written by Robert Lamm and recorded by the group Chicago for their 1972 album Chicago V.
So, the final answer is the group Chicago.